So I've beaten all 16 gyms, as well as the Elite 4 and the upgraded Elite 4. Without going to Red and beating maybe 3 of his pokemon each battle, where is a good place to train? I'm not talking about my level 70s, but more along the line of my level 40s. I usually train with whatever's available in Kanto, but when that's juiced and nobody wants a rematch, it can get kind of tough beating the same wild pokemon over and over for the same measly experience. 
 
tl;dr: Where do you train your medium level pokemon after you've beaten all the major obstacles in the game?

Keep spamming the Elite Four. Have a high level pokemon to help out too.

" Keep spamming the Elite Four. Have a high level pokemon to help out too. "

Battle frontier doesn't gain you any experience.
 
Either go with Elite Four (20 times is hyperbole and they'll have higher levels this time around anyway I think) or the fighting dojo in Saffron once you've gotten the phonenumbers of the gymleaders. Otherwise you're at that point in Pokemon games where grinding just takes time I think, no real way around it.

" Battle frontier doesn't gain you any experience.  Either go with Elite Four (20 times is hyperbole and they'll have higher levels this time around anyway I think) or the fighting dojo in Saffron once you've gotten the phonenumbers of the gymleaders. Otherwise you're at that point in Pokemon games where grinding just takes time I think, no real way around it. "